Create new scss file for xterm

What does this MR do and why?

This MR makes it easier to fully adopt Vite CSS handling. It does so by creating a named entrypoint for xterm CSS.

This MR is a blocker for Move CSS compilation to Vite Ruby (!152044 - merged).

How to set up and validate locally

1. Open any job log
2. You should see proper styling applied to the output